Displaced Syrians who fled from the Islamic State (IS) group's stronghold of Raqa arrive in the village of Hawi al-Hawa (AFP Photo/Delil souleiman)

The United Nations on Wednesday urged those fighting in Syria's Raqa to remember "obligations to protect civilians," voicing concern over the tens of thousands of people still trapped without access to aid.

An estimated 10,000 to 25,000 people are stuck in the city, the Islamic State group's main stronghold, said UN spokesman Stephane Dujarric, who noted that "exact figures remain difficult to verify due to the situation on the ground."

"Access to Raqa is not currently possible for the UN, due to the fighting on the ground," Dujarric said during his daily briefing.

"We remind all military actors of their obligations to protect civilians and ensure humanitarian access, in line with international humanitarian law."

In July humanitarian organizations distributed aid, including food and medications, to some 263,000 people out of at least 46 sites within the Raqa region, Dujarric said.

The Syrian Democratic Forces, a Kurdish-Arab alliance backed by an international coalition battling IS, entered Raqa city two months ago.

But the force's progress has been hindered by IS which is fighting hard to hang on to the de facto Syrian capital of the "caliphate" it self-proclaimed in June 2014.

The SDF is backed by the US-led coalition battling IS in Iraq and Syria, which has supplied weapons, air cover and a limited number of troops.

The fight for Raqa has prompted tens of thousands of civilians to flee, with many wounded by crossfire or IS-planted explosive devices during their escape.